In the drilling of a subterranean rock formation, tubing such as a drill stem assembly or drill pipe string conducts drill mud or drilling fluids downhole to tools such as mud motors or a drill bit. Drilling fluids are circulated downhole through the tubing for a variety of purposes including carrying drill cuttings up an annulus about the tubing and to the surface for screening, reconditioning and recirculation. In drilling with coiled tubing, mud motors are powered by the flow of drilling fluids and are used to rotate the bit. In conventional rotary drilling, the drill pipe string is used to rotate the bit. In either case, there are one or more connections in which a screen can be installed to intercept debris and other oversize material carried by the drilling fluids which could interfere with the downhole tools. Ideally, a retrievable screen is used such that the screen may be occasionally removed, cleaned and reinstalled without having to extract the tubing from the hole.
A variety of retrievable drill pipe screens have been disclosed in the prior art. Some prior art screens are supported on a mounting collar that is sandwiched between interconnecting uphole pipe and downhole pipe of a drill pipe string. Such screens comprise a screen support ring that is adapted to releasably engage the mounting collar and from which a screening member depends downhole into the bore of the downhole pipe. Conventional means for enabling retrieval of the screen from the drill pipe string include a fish neck connected to the screen support ring and which extends uphole therefrom. The fish neck is engageable by a conventional overshot fishing tool for pulling the fish neck and screen free from the mounting collar. Sometimes the fish neck is connected to the screen support ring by leg support members that are welded to the fish neck and the support ring in a so-called “pyramid” or “spider leg” configuration. Alternatively, as disclosed in U.S. Pat. No. 4,495,073, the fish neck may be connected to the screen by a bridging bar secured in transverse relationship to the screen support ring onto which the fish neck is centrally attached.
Applicant has found that such prior art means for attaching a fish neck to a screen may restrict flow of fluid into the screen and thereby impede the rate of flow of fluid down the drill stem. Furthermore, such restrictions do little to alleviate wear due to fluid flow conditions including vortices in the fluid flowing through the screen and also that due to pressure difference thereacross which accentuates wear. Wear can result in various failure modes including: of the screening member, of the attachment between the screening member and the screen support ring, and of the attachment between the fish neck and the support ring.
There is, therefore, a need in the art for an improved retrievable pipe screen.